Lucia Castillo dreams of being a hero like her grandfather. But to the people in their Texas town, he was just a strange old man who dressed up as a knight and claimed to save the world from monsters only he could see. Now years later, when Lucia and her best friend--and trusty squire--Sandro discover the town mayor is secretly a shapeshifting beast of Mexican lore, her parents think she's imagining things like her "Abuelo Loco." Only Lucia, wearing her grandfather's magical helmet, can see the hidden threat. Can she and Sandro prove others wrong and stop the mayor from unleashing evil on their town--and beyond?